Any reason why `emit` says it has to be ran from a...

# android

[Nikola Milovic]

Any reason why emit says it has to be ran from a coroutine body ( I am well aware it is a suspend function)?

import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.flow

class FirebaseDataSource (private val firebaseFirestore: FirebaseFirestore) : NetworkDataSource{

    fun fetchMenuItems(): Flow<Food> = flow{
        firebaseFirestore.collection("menu_items")
                .get()
                .addOnSuccessListener { documents ->
                    try {
                        for (document in documents) {
                            val doc =...
                            emit(doc) //error here
                        }
                    } 
                }
    }

In every example I've seen online they have it just inside the flow {} Eg.

fun simple(): Flow<Int> = flow { 
    for (i in 1..3) {
        delay(100)          
        println("Emitting $i")
        emit(i)
    }
}

[tseisel]

That's probably an issue with context preservation: you are probably emitting after the end of the flow block has been reached.
Two suggestions here:
1. write an equivalent of get() that suspends. You can do that using suspendCancellableCoroutine.
2. Use callbackFlow.

[akatkov]

should be able to call await after the get() to convert the Task to suspending e.g. val documents = firebaseFirestore.collection("menu_items").get().await()

[akatkov]

if not, then you can write your own suspendCancellableCoroutine as mentioned above

[Mark Murphy]

The lambda expression that you are passing to flow is a coroutine body. However, the OnSuccessListener lambda expression is not part of that coroutine body. That converts into a separate function on a separate object and lives separate from the Flow.

callbackFlow() would allow you to adapt this callback-based API to a `Flow`: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/callback-flow.html

Or, there seem to be some existing Firebase-with-coroutines libraries, such as https://github.com/brotoo25/firestore-coroutines.

[Michal Harakal]

Sample of AWS login with 2 different callbacks

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